Rituals are a set of skills that have item and time requirements, but are relatively cheap MP-wise. On the other hand, they are by their nature somewhat unpredictable in their output; improving their skill rank or increasing the items used improves your chances, but never removes the element of chance entirely. Unlike CRAFT items, rituals cannot be modified, improved, or altered other than in ways they specify.
Whenever the players vote to perform a ritual, the QM tells how long it'll take/ how much it'll cost in MP.
AKEMI HOMURA has learned RITUAL (PROTECTION), RITUAL (PURIFICATION), and RITUAL (SUMMONING) in Loop 2 from books purchased at a 'real magic shop' in Mitakihara, and RITUAL (MAGICAL MENDING) from a summoned guidebook in Loops 2 and 3. The only ritual she has performed thus far is Magical Mending.